Variable Interest Entities - Summary of Variable Interest Entities Information (Detail) - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2018	Dec. 31, 2017
Variable Interest Entity [Line Items]
Total Assets	$ 220	$ 230
Total Liabilities	1	4
LIHTC Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	5,955	6,003
Total Liabilities	2,589	2,943
Maximum Exposure to Loss	1,567	1,561
KCC Indirect Investments [Member]
Variable Interest Entity [Line Items]
Total Assets	21,088	26,817
Total Liabilities	376	292
Maximum Exposure to Loss	$ 129	$ 153